Hi Tom and all,
> To protect my 830's, I now have 'protection boxes'
> in series with the RX Antenna line (available through
> the transverter plug). These boxes consist of a
> series 2W carbon composition (non-inductive)
> resistor followed by a pair of 4 high PIN diode
> stacks of opposite polarity. Any low capacitance
> high speed switching diode should also work.
PIN diodes are actually slow diodes, not fast. They "hold" in-to or
out-of conduction for longer periods than normal switching diodes.
That's why a small dc bias "holds" them on or off for fast changing
RF. The diodes just don't see the RF, because they are too slow.
They are sensitive to dc bias, not RF voltage, when operated over
their normal frequency range.
You'd never want to use an actual PIN diode for an RF clamp
unless you apply dc bias before you apply the RF. By the way,
these diodes are called PIN (and engineers mostly hate the term
"pin" because it is a P I N diode). The name comes from the
intrinsically pure silicon wafer they are made from. That's why they
are expensive, they come from a pure and thicker than normal
silicon wafer.
The best diode for a self-biased clamp are just regular fast
switching diodes. I use 1N914's or fast switching diodes when I use
them, and if power levels are high I use a small 12 volt light bulb
between the back-to-back diodes and the antenna lead.
Do NOT use PIN diodes, and do not use Schottky devices (they
breakdown at too low a voltage)!
Also remember these diodes can be the source of IMD and TVI! If
they are clamping, they are mixing and generating harmonics.
> hard to find these days but other non-inductive
> resistors are available.
The other resistors are actually better. Carbon has a tendency to
age down in value. Being a semiconductor, carbon resistors can
"short". If you use a metal-film resistor the failure mode is always
open.
That's a trick in designing stuff. You look at the resistor in a critical
application, and decide whether a short or open would be the better
failure mode. If you want a resistor that is more likely to fail
shorted, you use a heavy carbon comp. If it can never fail shorted
without hurting something, you use a metal type.
I use a series lamp (you may have to parallel a few to get the
resistance low enough) because it increases resistance with power
and it acts like a fast fuse. If the diodes are really getting whacked
with RF a series lamp reduces harmonics radiated back into the
RX antenna more than a fixed value resistor would.
If you have trouble with BC harmonics and stuff, you might want to
series connect a string of back-to-back parallel diodes and
increase the clamping voltage.
73, Tom W8JI
w8ji@contesting.com
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